As well known, gas turbines or in general machineries configured to extract useful work by means of elaborating the flow of hot gases, include components, such as airfoils, deployed to such function. Airfoils, during operation of the machinery, need to be cooled in order to maintain the temperature of their parts within acceptable limits. For this reason, airfoils generally comprise longitudinally extending inner channels, or ducts, which are configured to receive cooling air. Usually, the airfoil comprises a plurality of inner channels connected to each other in a serpentine-type configuration. In particular, the cooling air enters in the cooling circuit in a first channel which is located in the proximity of the leading edge, which is the portion of the airfoil with high external heat load, and exits the circuit through a cooling channel which is in turn located in the proximity of the trailing edge.
Inside the channels, turbulence promoters or turbulators are means commonly deployed in order to generate turbulence near the inner walls of the cooling channels, thus enhancing the thermal exchange between the cooling flow and the airfoil and therefore improving the cooling performance.
Turbulators are generally provided in the form of rib-shaped elements disposed on the inner walls of the cooling channels.
One of the parameter which affects the heat exchange between the airfoil and the cooling fluid is the angle formed between the direction of the cooling air flow, which follows the longitudinal axis of the airfoil, and the turbulator. For airfoils having a straight longitudinal axis, the ribs maintain a constant inclination along the vertical axis and such angle is constant throughout the cooling channel.
However, in order to improve the turbine efficiency, the airfoil may be three-dimensional shaped meaning that its longitudinal axis may be curved and deviating from the vertical axis, which is the component design axis and it is generally referred to the placement of the gas turbine. Such curvature is usually mostly pronounced in the proximity of the leading edge portion of the airfoil.
As a consequence, arranging the ribs along the inner wall of the cooling channel with a fixed inclination in respect to a vertical axis results in a poor cooling performance, since the areas where the longitudinal axis is curved are characterised by a reduction of the angles formed by the ribs and the direction of the cooling air flow. As indicated above, such angle is a driving parameter for the efficiency of the cooling system.
In addition, still according to known airfoils, the distance between subsequent ribs, or rib-to-rib pitch, which is another driving parameter for the heat transfer, is kept constant along the vertical axis. Similarly, such known geometry leads, for airfoils having 3D curved shape, to a change in the rib-to-rib pitch, commonly known as P. In particular, this leads to even higher pitches and, as for the effective rib angle, further decreases the heat transfer in those affected regions.
Therefore, the standard design approach provides inadequate cooling particularly for airfoils having curved profile. Furthermore, the standard approach leads to inhomogeneous cooling along a curved duct, since in the area with the most curved shape the heat transfer is lower compared to the straight portion.
A possible counter measurement for a standard design would be to increase the cooling air consumption. However, the geometry of the turbulators deployed inside the cooling ducts would remain the same and therefore this would result in less cooling performance in the 3D shapes area compared to the straight portion and the inhomogeneous cooling along the duct will still be present. In addition, increasing the cooling air flow, such to maintain the metal temperatures in the curved portions of the airfoils within predetermined valued, would inevitably reduce turbine efficiency and power.